2. Agenda
OFDMA
WiMAX Frame
Sub channels and Permutations
Smart Antenna Systems
Summary of WiMAX PHY features
Beyond 4G
2 WIMAX Series – Understanding PHY
3. Lets understand fading in wireless
communications
Frequency selective fading ( Fading (Delay Spread)
MULTIPATH FADING CHANNELS
Delay Spread)
Delay Spread)
Time Selective Fading ( Doppler ) Multiple copies of signal arrive at
the receiver, offset in time
Beyond 4G
3 WIMAX Series – Understanding PHY
4. WiMAX Addresses Fading
Frequency selective fading (
Fading (Delay Spread) Delay Spread)
OFDMA
Diversity Schemes
Multiple Antenna Systems
Multiple copies of signal arrive at
the receiver, offset in time Cyclic Prefix ( ISI )
Forward Error Correction
Beyond 4G
4 WIMAX Series – Understanding PHY
5. Concept of OFDM Subcarrier
In OFDM available OFDM is robust to fading
bandwidth is divided High spectral efficiency
into N (large number Eliminate Inter-symbol
of) orthogonal sub- Interference (ISI) by the
carriers introduction of the cyclic
Each sub-carrier is prefix (CP)
individually modulated Sensitivity to frequency
(QAM) at 1/N total offset
data rate
Higher Peak to average
FFT implementation power ratio(PAPR)
(fast and efficient)
Beyond 4G
5 WIMAX Series – Understanding PHY
6. OFDM vs. OFDMA
Subcarrier
SC – Type: Data, Pilot, null
(guard and DC)•
– Number: 128, 512, 1024,
OFDM 2048 (at least one)
OFDMA
Subcarrier Subchannel
– A set of sub carriers forms a
subchannel
– The sub carriers may and
may not be adjacent
Subchannels – Distributed allocation
(diversity)
– Contiguous allocation
(selectivity)
Beyond 4G
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7. OFDMA Advantages
•Orthogonality of users are maintained
A A E E using large symbol duration and cyclic
prefix
SUBCHANNEL
B B B B
C C C C •Users concentrate transmit power over
partial bandwidth
D D D D
•High granularity in resource allocation
TIME & power control
•Inherent resistance to multi-path
• Users/bursts are scheduled (OFDM)
across both frequency
(subchannels)and time (symbols) •Flexible Sub-channelization
• Per burst modulation, coding •Distributed (pseudo-random)
and TX power permutations –frequency diversity
• Higher granularity in resource
•Contiguous (clustered)
allocation
permutations -> selectivity
• More degrees of freedom in
scheduling •Scalability of Channel Sizes
• Improved fairness and QoS
Beyond 4G
7 WIMAX Series – Understanding PHY
8. OFDM Symbols and Cyclic Prefix
Pilots used for channel Use Cycle Prefix (CP) as
estimation and tracking guard interval
Interleaving (bits Prepend segment of
distributed across tones) OFDM symbol –>
and error correcting overhead
facilitates recovery under
frequency selective fading CP duration > max delay
spread
or narrow-band
interference Eliminates ISI
Retains orthogonally
between sub-carriers (ICI)
Beyond 4G
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9. OFDMA Frame Structure
CQICH Uplink and Downlink is
s
divided into time zones
UL-
Different Permutation
UL BURST 3
S
MAP
DL BURST 4
U
B
P
ACKCH
’s for downlink and uplink
C
R
H
A
E
A DL-
– DL PUSC,AMC etc
N
– UL PUSC,AMC
M MAP
N P
B C
E e
L DL BURST 1 D UL BURST 2
L ri
S
E M
A
o
d
– Activation STC/MIMO
R
CDMA Codes
i
DL BURST 3 A
N
C
R
G
a
DL BURST 2 I
N
n
g
UL BURST 1 – Special allocation in
G
. uplink for ranging and
BW Request
S1 S2 Sn
DOWNLINK TTG UPLINK
RTG
Fast Feedback/CQICHs
– SS measured CINR
Beyond 4G
9 WIMAX Series – Understanding PHY
13. Permutation schemes in UL : PUSC
Tile Structure
3 SYMBOLS
• 4 Sub carrier X 3 Symbol
• 4 Pilot + 8 Data
Pilot
Sym
3
Sym
3
Sym
3
Sym
3
Beyond 4G
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14. PHY Data Rates
Without MIMO and overheads
Beyond 4G
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15. Advanced Antenna Systems : STBC
STC gains vary from 5-10 db depending on channels condition
Beyond 4G
15 WIMAX Series – Understanding PHY
16. MIMO B : Spatial Multiplexing
SM can double throughput in theory by require very high
CINR conditions of uncorrelated signal to achieve full
advantages. 30% sector improvement in loaded conditions.
Beyond 4G
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17. Beam forming
Beamforming can provide additional 5-6 db gains in downlink
improving overall network quality.
Improves sector capacity by 20 to 25%
4Rx in uplink provides additional 3 db gain
Beyond 4G
17 WIMAX Series – Understanding PHY
18. UL Collaborative MIMO
Collaborative Spatial Multiplexing for
two MSs with single transmit
antenna
Two MS transmit on the same
resource (sub channels)
For orthogonal zing
– Orthogonal pilot patterns (pattern
A, pattern B)
Multiple data detection
– Using multiple receive antennas
– Interference suppression: MMSE,
ZF
– Nonlinear interference
cancellation: SIC, PIC
Beyond 4G
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19. MIMO Summary
Parameters Gain in Link Gain in Comments
Budget Capacity
MIMO A 5-10* MIMO A in low SINR condition
improves channel fading.
Switching between MIMO A and B
is automatic and controlled by
BTS.
MIMO B Nil 30% in Average improvement of 30% in
average full load , customers higher CINR
or uncelebrated signals will take
advantage of MIMO B
DL-Beam 5-6 db in 20 to 25% BF helps in improving capacity
Forming Downlink over MIMO and improve network quality.
4th order 3 db over 2nd Nil Improves cell radius by 3db in
Diversity order uplink.
diversity
*STC + 2 Rx in receiver with possibility MRC combining
Beyond 4G
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20. Mobile WiMAX PHY Summary
Feature Support
Scalable Channel TDD , 5 and 10 MHz , 20 (optional)
Bandwidth
Frame size and cyclic 5 ms , 1/8
prefix
Sub-carrier allocations PUSC,AMC,FUSC
Error correction and CC, CTC with HARQ
HARQ
Modulation Downlink :QPSK,16QAM,64QAM
Uplink : QPSK, 16QAM
Uplink Control channels Ranging , CQICH’s , ACKCH
MIMO Downlink : 2x2 , STC , SM
Uplink : 1X2 , Virtual SM
BeamForming AMC or PUSC
UL channel sounding
Beyond 4G
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21. Thanks
Beyond 4G
21 WIMAX Series – Understanding PHY
